Optical fibers, including communication fibers with low-impurity levels are made from preforms. A preform can be made by several methods. In general, these methods are divided into three categories: hydrolysis (reaction with H2O), oxidation (reaction with O2) and sol-gel (reactions with a suspension of silica).
For example, the hydrolysis method is accomplished by flowing SiCl4 vapor into a hydrogen flame with the resulting “fumed” silica submicron particles collected on a rotating target. Various dopants may be added to the flame (e.g., GeCl4, POCl3). The cylinder produced in this manner is treated at a temperature near 800° C. with an atmosphere of SOCl2 to reduce the OH content of the glass. Subsequently, a transparent glass preform is made by fusing the particles at a temperature of 1500° C., a process referred to as sintering. The processes involved in making preforms in accordance with the oxidation method involve deposition and sintering steps carried out inside substrate tubes. Sol-gel processing has been investigated primarily for the production of silica tubes used to overclad higher purity core and inner cladding regions of preforms.
A preform made by any of these techniques is drawn into an optical fiber. Drawing involves heating one end of the preform to the softening temperature and pulling it into a fiber at rates as high as 20 meters/second, or greater.
With the advent of more complex fiber designs there is a need to better control the refractive indices of the fiber's core and cladding, as well as the overall refractive index profile. Also, some fibers have additional cladding layers in which the refractive index has to be precisely controlled. Thus, it is necessary to add dopants to the primary glass constituent SiO2, to change its refractive index, thus allowing control of the fiber waveguide design. Index-raising dopant ions (e.g., germanium, and aluminum) and index-lowering dopants (e.g., boron and fluorine) are introduced into the reaction stream as halide vapors carried by oxygen or an inert gas. The incorporation of the dopant ions in either the hydrolysis or oxidation processes is controlled by the equilibria established during dopant reaction, deposition, and sintering.
Variations in the refractive index due to these equilibria manifest in sawtooth patterns of refractive index in the cladding of the depressed cladding fiber and depressions in the refractive index in the center of the fiber. For this reason, production of depressed cladding fibers with highly accurate index profiles from drawn preforms poses a challenge. The addition of dopings of active materials into the preform, e.g., rare earth elements such as Er, Nd, Tm or Pr, further exacerbate the problem. In fact, for doped depressed cladding fibers index tolerances of up to 20% are common. For more information on the technology of making performs and drawing fibers the reader is referred to Erbium-Doped Fiber Amplifiers Fundamentals and Technology by P. C. Becker, N. A. Olsson, and J. R. Simpson, chapter 2 (Optical Fiber Fabrication), published by Academic Press, pp. 13–42 and references therein.
A short-pass fiber designed in a depressed cladding fiber is particularly susceptible to the above tolerance limitations. Specifically, short-pass fibers of the type described in U.S. patent application Ser. No. 09/825,148 filed on Apr. 2, 2001 and in U.S. patent application Ser. No. 10/095,303 filed on 8 Mar. 2002 are highly sensitive to tolerances in refractive index as well as radii of the core and cladding layers. That is because the cutoff wavelength and the roll-off loss curve of such short-pass fibers depend on these parameters.
Unfortunately, the prior art techniques are not capable of consistently delivering depressed cladding fibers within the refractive index and cross-sectional tolerances required for short-pass fibers with well-defined cutoff wavelength and roll-off loss curves. In view of this, it would be an advance in the art to provide a method for fabricating short-pass fiber.